Optical technologies (photonics) are increasingly being used in particular in the fields of laser material processing, for example in nanotechnologies or for the precise manipulation of biological material. Of particular interest and importance is the interaction of laser light and material with very short laser pulses (10−12s to 10−15s) which is based on fundamentally new physical processes and can be advantageously used for high precision material processing.
Conventional ultrashortpulse laser systems are constructed and optimized to generate pulse sequences having a constant pulse repetition rate (i.e. a constant timing, or a constant temporal distance between two subsequent pulses) and having constant pulse energies. In particular when constructing pulse lasers, up to now these systems are being optimized for a fixed pulse repetition rate—i.e. the pulse sequence is generated asynchronously from the targeted application and with a constant pulse to pulse distance. The synchronization of the single pulse(s) of the continuously generated pulse sequence (also called pulse train) for example with an external process or with a target application in such systems is usually realized with the help of additional optical elements (optical switch) which can uncouple (separate) or select single pulse or a number of pulses from the already generated pulse sequence. This method is expensive and not efficient, since a large part of the generated laser energy is not used.
Apart from synchronizing the laser pulse emission with external events (i.e. the timing of the single pulses in the pulse sequence), another important parameter is the pulse energy. In conventional laser systems, pulses with desired energy can be generated by introducing variable losses in the generated usable beam. This is realized for example by inducing changes in the polarization in conjunction with an optical element with polarization-dependent transmission. However, with the available technical means this is only possible with relatively high temporal inertia. For example, consider a mechanical setup that can change the polarization of a laser beam to a desired orientation in 0.1 s and a pulse repetition rate of 10 kHz (pulses 100 microseconds apart). Up to now the energy of the pulses in a pulse sequence can therefore only be controlled over more than 1000 pulses, whereas for many applications it is desirable and necessary to change the energy on a pulse-to-pulse basis.
An attempt to control the properties of the pulses within the laser cavity in order to achieve stable pulse-to-pulse characteristics (usually a pulse sequence with constant repetition rate and small variation of the single pulse energies) is made for example with a Q-switched (Q-switching: cavity loss modulation) laser.
U.S. Pat. No. 5,982,790 describes a system and a method for reducing pulse-to-pulse energy and peak power variation in various types of pulsed lasers, in particular Q-switched lasers. The laser system described therein comprises a laser cavity having a lasing medium pumped by a pumping device for delivering pumping energy to the medium. The system includes further a detection device and a circuitry for determining the pulse magnitudes of laser pulses, such as the peak pulse amplitudes, the pulse energies, the pulse widths or other pulse metrics. The system comprises further a feedback mechanism which is in communication with the pumping device and which ensures pulse-to-pulse stability by increasing the pump energy when the pulse magnitude of the i-th pulse exceeds a mean pulse magnitude and decreasing it when it is less than the mean pulse magnitude. Alternatively, the feedback mechanism is in communication with the switching device which controls the variable loss factor of the Q-switch to achieve pulse-to-pulse peak and energy stability.
U.S. Pat. No. 6,339,604 discloses a pulsed laser system which includes a laser pump, a laser rod, a reflector interposed between the laser pump and the laser rod, through which energy from the laser pump enters the laser rod, an output reflector through which energy is emitted from the laser rod, a switch interposed between the laser rod and the output reflector and a control device. When closed, the switch causes energy to be stored in the laser rod and, when opened, allows energy to be emitted from the laser rod during an emission period. The control device allows a primary pulse emitted from the laser rod during the emission period to impinge on a workpiece and subsequently blocks or eliminates the workpiece secondary laser emission occurring during the emission period after emission of the primary pulse. The pulsed laser system is operated over a range of repetition rates, so as to cause laser energy to be emitted during a plurality of emission periods at each repetition rate. At least a portion of the laser energy emitted during the emission periods is directed toward the target structure. The switch is closed for a fixed, predetermined period of time prior to each emission period regardless of repetition rate of the primary laser pulse within the range of repetition range in order to store energy in the laser rod. The pump is operated continuously at constant power.
With pulse lasers in the nanosecond range that are based on the principle of Q-switch (control of the losses in the cavity) the pulse duration and the pulse energy cannot be adjusted independently from each other by the variation of the amplification time. That is to say, with increasing pulse energy the pulse duration is also increased considerably. Furthermore, these systems are directed at the generation of nanosecond pulses, rather than at the generation of ultrashort pulses in the pico- to femtosecond range.
In view of the above problems the object of the present invention is to realize an improved laser system in particular for the generation of short and ultrashort pulses which is capable of generating arbitrary, programmable and in particular dynamically synchronizeable pulse sequences.